Women After All: Sex, Evolution, and the End of Male Supremacy (11 page)

In palmate newts, the male’s tail ends in a threadlike filament. In their wild little world of European ponds, females watch courtship displays longer when the beau boasts a longer filament, and in experiments both filament length and display duration predict how many sperm masses a female will accept from a given male. In the tiny Trinidadian guppy, females that choose males with longer tails have more sons, and the sons have long tails, too. In the mandrill, a big monkey much more closely related to us than newts or guppies, males have bright red noses flanked by bulging iridescent pale-blue cheeks; Joanna Setchell showed that the more brightly colored the male, the more females hang with him, groom him, and mate with him, even after controlling for his rank on the dominance ladder. The list goes on and on, and even in our relatively pair-bonding species, male features such as broad shoulders, big pecs and biceps, chest hair, square jaws, and five o’clock shadows have been shown to figure in many women’s sexual choices.

So in pair-bonding species, female choice remains important, but it follows a different path. First, sexual selection is more balanced; both male and female may be attracted to certain physical traits. The roseate tern is a sleek foot-long flier seen around the world. Both sexes are white underneath, with gray wings, a black cap, and sometimes an orange beak. Both have swallowlike tails with lengthy outer streamers, and each sex fancies long streamers in the other. In one population in Queensland, Australia, a dearth of males led some females to pair with other females, while those with longer streamers got the hard-to-find, coveted males. But despite the pair-bonding, the similarity of the sexes, and the fair balance of parenting, there is one dramatic asymmetrical behavior: courtship feeding. The female
tern sits patiently while the courting male goes a-fishing—but not for a lazy afternoon. The busy boy fetches her gift after wriggling gift of silvery prey; the way to her heart is through her stomach. He is, of course, also proving that he can bring home the anchovies for their future young.

Thousands of species of birds do courtship feeding. Male cardinals bring seeds and put them
in
the female’s mouth. Gull suitors bring up a half-digested blend of fish and squid, laying the prize grandly at the lady’s feet. Kestrels and owls offer freshly killed mice or lemmings to their perching hoped-for brides, while in other raptors males actually pass the bloody package to the female during flight, dropping the bird or snake as the agile mom-to-be flies under him. Despite the equality of parental effort after the eggs are laid, the female still has to manufacture and lay them, and this makes her a goal worth working for—and feeding her improves her nutritional condition and that of the young. It’s not until a species gets on the jacana plan that males actually do much more parenting than females. In other species, males have to prove themselves—as foragers, hunters, providers, fathers, sex objects, or fighters. But even in our postmodern, postfeminist world, men usually pay the restaurant bill on dates, our version of courtship feeding.

These rituals do not work by magic; in birds, at least, they are wired into the brain. In the ringdove, bowing and strutting by the male and cooing by both sexes stimulates the male’s and female’s gonads, a song-and-dance routine that lasts for days and gets both ready for mating. First the male chases the female and she withdraws, but at a certain point he sits on the nest and gives a softer coo, which she returns, further stirring her own hormones. This goes back and forth until, if she accepts him, she mates with him and they raise the young together.

None of this, of course, is conscious or deliberate, as it might be if human males and females were doing some version of it. It’s hormonal; it’s instinctive; it’s evolved. But, conscious or not, our version
evolved, too. Men woo in a variety of ways, but only those that persuaded women have descendants around to tell the tale. True, in many cultures and much of the past, men had to persuade women’s families, but as we will see, women’s choices mattered then as well.

In fact, it is not far from the truth to think of the whole of evolution—at any rate, evolution since sex was invented—as a long, slow, meticulous breeding experiment done
on
males
by
females. Of course, if a female bird or mammal wants to reproduce, she has to choose some male or other, and unfortunately it may not be a dazzling show but superior size and brute force that conclude the issue (as happens the other way round in jacanas). Still, a female has to be willing and ready, and not every male can bring about that readiness.

Even a female rat, called a doe, has to be lured into lordosis, a dramatic reflex triggered by some males. First the doe has to want the buck. Then she darts toward him and flirtatiously hops away, or just skirts him in a move known as a run-by. She’ll do this, wait a bit nearby, seductively wiggle her ears, and repeat the sequence. She might even intercept his moves on another doe with her own moves on him. If this seduction succeeds, he’ll begin to mount her, and the touch of his legs on her flank will trigger the last enabling reflex. She’ll stretch her lower back and lift her butt, so he can easily enter where she now wants him to be.

This is estrus, instinctive and cyclical. At a certain point, when the sequence has gone far enough, you can trigger lordosis with your finger on the doe’s flank, but that’s not the real world. As with the ringdove, brains and hormones are involved. You can make it happen by stimulating part of her hypothalamus. Giving her estrogen puts the circuits, including the nerves in her pelvis, on high alert. And the response can be contravened: in a famous study brilliantly called “Sex with Knockout Models,” Emilie Rissman and her colleagues, using mice, showed that females whose gene for the
estrogen receptor is knocked out attack males when they would normally be receptive and fail to show lordosis when their flanks are stimulated, even after estrogen injections. These built-in systems are just part of the meaning of instinct. But there is also choice. All right, the doe isn’t sitting around with her friends discussing how good-looking a buck is, how much money he makes, or how often he showers. But rat and mouse do have their ways of choosing, and eons of evolution have honed their instincts for acceptance or rejection according to male quality.

Creatures like peacocks are known as tournament species, because they gather on a breeding ground where males show off while females pick and choose. Female choice here is intense. But unlike in jacanas or pair-bonding species, the females aren’t probing for possible doting dads; doting is not on the dance card of the chauvinist males available. So the females are after three things: sex appeal for their future sons; such high quality that the male can waste energy and risk death to build ornaments and show them off in dazzling displays; and, significantly, the ability to intimidate and defeat other males.

This is where the tournament comes in. In many species, the males are like jacana females: not so much ornamented as big and tough, domineering and dangerous, to females as well as to other males. Black grouse are a classic example; males congregate in a spot that has little to recommend it except that females know they can find mates there. The males, almost two feet long, dark gray with iridescent blue highlights and dramatic bright red eyebrows, belt out a distinctive mating song and swagger to exhaustion or until a female—smaller and a drab brown—chooses them. These jamborees happen at dawn in the spring and can involve hundreds of birds. But Anni Hämäläinen and her colleagues, in a 2012 study, showed that it is fighting performance in males that the females are mainly after. Similarly, in the Uganda kob, an antelope around three feet high, males are taller and heavier than females and sport horns.
Many individual males defend territories, where they try to maintain harems; others collect on mating grounds, where three to seven males hold the center and monopolize sex with many more females.

You don’t, however, need crowded tournaments to have intense male competition. Consider the majestic red deer, studied for decades on the Isle of Rum, off the west coast of Scotland, by zoologist Tim Clutton-Brock and others. Close kin to the American elk, they are equally imposing and very successful, ranging over Europe and parts of Asia and North Africa. Stags can weigh over 500 pounds and hold aloft an intimidating rack of antlers; hinds are bareheaded but can weigh 350 pounds. Mature stags maintain harems and peak in breeding at age eight. During the autumn rut, rivals challenge them. The stags walk in parallel to assess each other’s body and antler size. They often bellow, launching a loud hoarse groan. If they fight, the clash of antlers is colossal, and they can hurt each other badly. Victors gain or keep their harems and approach and mount hinds, even beginning thrusting with several hinds before completing the act. A hind may walk away. The rut is exhausting; stags may shed 20 percent of their weight. It’s no surprise that males age out of all this within a few years.

But in these stags and for other deer and antelope, are the horns true weapons, in Darwin’s original sense, or just costly ornaments advertising quality? Probably both. Certainly antlers and horns impress choosy females, but they also intimidate rival males. If the rival is of an age and size to do more than put on a show, these accessories can inflict serious damage. But they often don’t make sense purely as weapons; they are designed not so much to harm as to inspire. So ornamentation clearly matters. Size itself is vital for male defense, but it is also clearly sexy in many species, and in some species females can “see” it in the dark. The croaks we hear in a kind of chorus at night around a pond are belched out by male frogs broadcasting their presence to females. At the same time, they are declaring their size. Although other factors matter too, the more
basso the croak, the larger the frog, and the more likely the female is to approach when she hears it.

Nowhere, however, do size and force matter more than in the elephant seal. To be anywhere near them on a beach is to know how puny we humans are. The relatively svelte females are only eleven feet long and a comparatively modest fourteen hundred pounds; males are three feet longer and more than triple that weight. They have huge swellings on their snouts to help them belt out colossal roars, and they charge across the beach and slam into each other at astounding speeds. They sink their teeth into each other’s necks and shoulders, bloodying their chests until they are smeared red and pocked with wounds that leave permanent scars. These epic fights take no notice of any thing or creature in the way, including females and even pups that could be their own, which they sometimes crush to death, like trucks flattening squirrels. Cassowary and jacana females may be as large and cruel on their own scales, but the scale of elephant seals is titanic.

Yet every pound of the male elephant seals’ bulk and every thrust of their brutality are worth the cost and risk to them, because the potential reward is huge. As Burney Le Boeuf showed many years ago, 4 percent of the males get 85 percent of the sex. Of the other 96 percent, most get none at all—if they survive. But a winning bull can inseminate fifty cows in a season, fathering so many pups that he can afford to squash a couple while defending his benefits. Plan B is to be replaced at the top of his massive harem by a rival and breed no more.

It’s easy to see that these kinds of odds and differences in success could produce rapid evolution, at least for a time. Size can’t increase forever, because there are physical and physiological constraints. And the shared genes of males and females set a limit on the divergence between the sexes, especially in birds and mammals. But the process of getting there, of diverging until you have pressed against those limits and then staying there, can follow an obstinate evolutionary
logic. Cassowaries and jacanas are exceptions that prove the rule; they show decisively that it’s not about masculinity and femininity, it’s about parental investment and sexual selection. In mammals, however, the sex that invests more in the offspring is usually the female, which means that most species with divergent sexes are the opposite of jacanas.

In a classic paper, “Mammals in Which Females Are Larger Than Males,” Katherine Ralls founded a field of research. She began by noting that females are probably larger than males in most species of invertebrates, many fish, and some amphibians and reptiles. Among birds, she reminded readers of jacanas and phalaropes, but she also said that few biologists seem to be aware that the females-bigger phenomenon occurs in mammals, too. She found eighty-four examples.

Among them were hippopotamuses, okapis, several small deer and antelope, mongooses, hyenas, certain seals, whales and dolphins, several mice and shrews, quite a few bats, a couple of flying squirrels, some hares and rabbits, chinchillas, Burchell’s zebra, and a number of small marsupials, including the Tasmanian devil. Among our relatives the primates, she mentions South American monkeys, but we now know that females are larger and dominate males in many species of Madagascar lemurs, as we’ll see in the next chapter. Ralls also found that in almost all of the eighty-four species, females exceed males in length by less than 10 percent, with an average of less than 5 percent. This is nothing like the size difference favoring females in many insects and fish or even in cassowaries and jacanas. And it’s nothing like the excess males show in many mammals, elephant seals being the most extreme.

However, when we consider that men are only around 7 percent taller than women, the female size advantage in all those other species doesn’t seem so trivial. The golden hamster, for instance, reverses our own ratio. And in what is often called the largest species
that ever lived on earth, the blue whale, cows are bigger than bulls, the record female stretching almost one hundred feet.

Ralls thought, and recent research confirms, that the reasons for larger females are complex. Some female-larger mammals have a lot of fathering help from their male mates, but most don’t. Males do defend the young, but this happens in many male-bigger species too. Nor are larger females more aggressive than smaller males; males usually fight more, as in other mammals. In many of these species, the smaller males have typical Darwinian weapons and ornaments specialized for fighting—against other males. Ralls speculated that smaller size gives the males of some species (whales, for instance) greater mobility in mating. So selection may not so much have made females large as males small—female choice again, but in these cases favoring agility, not bulk.